1. Field of the Invention
The present invention relates generally to semiconductor light sources such as LEDs and VCSELs, and more particularly to a strained layer semiconductor laser having an emission wavelength of at least 1.3 xcexcm.
2. Description of the Prior Art
Vertical-Cavity Surface-Emitting Lasers (VCSELs), Edge Emitting Lasers (EELs) or Light Emitting Diodes (LEDs) are becoming increasingly important for a wide variety of applications including optical interconnection of integrated circuits, optical computing systems, optical recording and readout systems, and telecommunications. Vertically emitting devices have many advantages over edge-emitting devices, including the possibility for wafer scale fabrication and testing, and the possibility of forming two-dimensional arrays of the vertically emitting devices. The circular nature of the light output beams from these devices also make them ideally suited for coupling into optical fibers as in optical interconnects or other optical systems for integrated circuits and other applications.
For high-speed optical fiber communications, laser or LED emission wavelengths in the 1.3 xcexcm through 1.55 xcexcm region are desired. Standard silica fiber has zero dispersion near 1.3 xcexcm and has a minimum loss near 1.55 xcexcm. The need for semiconductor lasers emitting in this wavelength region has spawned worldwide development of such lasers. Group III-V semiconductors which emit light in the 1.3 through 1.55 xcexcm region have lattice constants which are more closely matched to InP than to other binary III-V semiconductor substrates, for example, GaAs. Thus, essentially all commercial emitting lasers emitting at 1.3 through 1.55 xcexcm are grown on InP substrates. These lasers are edge-emitting lasers which, unlike VCSELs, do not require high-reflectivity Distributed Bragg Reflectors (DBRs) to form their optical cavities.
Unfortunately, it has proven difficult to produce effective DBRs on InP substrates. The available materials which lattice match InP have produced mirrors which are extremely thick and lossy and have thus not resulted in efficient VCSELs.
VCSELs or Surface Emitting Lasers SELs whose current flow is controlled by lateral oxidation processes have show the best performances of any VCSELs in terms of low threshold current, high efficiency, and high speed. All such xe2x80x9coxide VCSELsxe2x80x9d have been fabricated using AlAs or AlGaAs layers which were grown on GaAs substrates and later oxidized. Thus, one would want to utilize a VCSEL structure such as is disclosed in U.S. Pat. No. 5,493,577, by Choquette et al. This VCSEL has the advantages of: (1) reduced mode hopping; (2) being temperature stable, and (3) testable in a modified silicon wafer tester. Unfortunately, this VCSEL structure will have an emission wavelength between 600 and 1,000 nm and thus falls short of the desired 1.3 xcexcm emission wavelength. Due to the availability of well-behaved oxidizable materials which may be grown on GaAs substrates and the straightforward capability of producing efficient high-reflectivity DBRs on GaAs substrates, when manufacturing VCSELs it is highly desirable to grow them on GaAs substrates.
The salient components of a VCSEL typically include two DBRs, and between them, a spacer which contains an active region having a length emitting material. The DBRs and active region form an optical cavity characterized by a cavity resonance at a resonant wavelength corresponding to a resonant photon energy. It has become a practice in the operation of VCSELs to detune the optical cavity to energies at about 25 meV lower than the peak transition energy by appropriate DBR spacing. Such xe2x80x9cgain offsetxe2x80x9d is used to advantage in reducing temperature sensitivity. This produces an emission wavelength which is appreciably longer than the peak transition wavelength. This practice, while inadequate in itself for increasing emission wavelength to 1.3 xcexcm from material grown pseudomorphically on GaAs substrates, does measurably increase emission wavelength. Even if this technique was incorporated with the teachings of the prior art, one would fall short of the desired 1.3 xcexcm emission wavelength.
While epitaxial growth of slightly-lattice-mismatched materials is undertaken routinely, materials which emit in the 1.3 xcexcm through 1.55 xcexcm region have lattice constants sufficiently removed from that of GaAs to make pseudomorphic epitaxial growth problematic. In this context, xe2x80x9cpseudomorphicxe2x80x9d means having a sufficiently low density of misfit dislocations such that lasers may be produced which have reasonably long lifetimes. The problems have been sufficiently great to cause researchers to abandon such efforts and resort to less desirable hybrid approaches to producing 1.3 xcexcm through 1.55 xcexcm VCSELs.
Thus, the production of VCSELs emitting at 1.3 through 1.55 xcexcm, wavelengths have been inhibited by either of two problems. The problems result from the fact that VCSELs require laser-quality active materials and high-reflectivity DBR mirrors. These two problems are:
(1) when InP substrates are used, growth of the light emitting active material is straightforward, but production of efficient DBRs is difficult and has not been effective; and
(2) when GaAs substrates are used, DBR production is straightforward, but efforts to grow laser-quality active material have been unsuccessful.
The following is a summary of the prior approaches which are relevant to the problem of producing 1.3 through 1.55 xcexcm VCSELs.
A 1.3 xcexcm edge-emitting laser grown on a GaAs substrate was reported by Omura et al., in an article entitled xe2x80x9cLow Threshold Current 1.3 xcexcm GaInAsP Lasers Grown on GaAs Substrates,xe2x80x9d Electronics Letters, vol. 25, pp. 1718-1719, Dec. 7, 1989. The structure comprises a layer having a high density of misfit dislocations, on top of which were grown thick layers of materials having lattice constants close to that of InP. Such lasers exhibit very poor reliability due to the misfit dislocations. Furthermore, this structure does not readily lend itself to integration with DBR mirrors.
The use of a layer having high-density misfit dislocations was also reported by Melman et al., in an article entitled xe2x80x9cInGaAs/GaAs Strained Quantum Wells with a 1.3 xcexcm Band Edge at Room Temperature,xe2x80x9d Applied Physics Letters, vol. 55, pp. 1436-1438, Oct. 2, 1989. The article states that pseudomorphic, i.e., nearly misfit dislocation free, growth of 1.3 xcexcm emitting material is not possible with GaAs barriers, i.e., GaAs substrates. This conclusion prompted the approach to incorporate a layer having a high density of misfit dislocations.
A 1.1 xcexcm emitting laser is reported in Waters et al., in an article entitled xe2x80x9cViable Strained Layer Laser at xcex=1100 nm,xe2x80x9d Journal of Applied Physics, vol. 67, pp. 1132-1134, Jan. 15, 1990. The laser utilized a single quantum well comprising In0.45Ga0.55As strained semiconductor material which has a greater thickness than its predicted critical thickness. Reliability tests are presented for 4000 hours of testing. To our knowledge, these are the longest-wavelength lasers produced on GaAs substrates which have survived such testing. In this article, even Waters recognizes the difficulty of creating a reliable device having an active region over the respective CT for the semiconductor material in the active region.
A strained quantum well emitting at 1.3 xcexcm is reported by Roan and Chang in an article entitled xe2x80x9cLong-Wavelength (1.3 xcexcm) Luminescence in InGaAs Strained Quantum-Well Structures grown on GaAs.xe2x80x9d Applied Physics Letters, vol. 59, pp. 2688-2690, Nov. 18, 1991. The quantum well was a short-period superlattice comprising alternating monolayers of InAs and GaAs. However, the quantum well had a thickness well above (1.78 times) the critical thickness, above which high densities of misfit dislocations exist. Thus, the structure is not viable for long-lived lasers and no lasers were produced from such a structure.
A compromise between GaAs and InP substrates is reported by Sahoji et al., in an article entitled xe2x80x9cFabrication of In0.25Ga0.75As/InGaAsP Strained SQW Lasers on In0.05Ga0.95As Ternary Substrate,xe2x80x9d IEEE Photonic Technology Letters, vol. 6, no. 10, pp. 1170-1172, Oct. 10, 1994. An In0.05Ga0.95As ternary substrate was utilized which has a lattice constant intermediate between those of GaAs and InP. The In concentration of the substrate was 5% of the group III material (2.5% of the total material) and the laser emitted at 1.3 xcexcm. The authors indicate that 1.3 xcexcm lasers will require an InGaAs substrate having about 25% or more In for the group-III material. Ternary substrates are unlikely to approach the availability, size and price of binary substrates such as GaAs.
James Coleman, in his book entitled xe2x80x9cQuantum Well Lasers,xe2x80x9d edited by Peter Zory, London, Academic Press, pp. 372-413, 1993, discusses the concept of critical thickness in strained layers lasers which utilize InyGal-yAs. As may be seen in FIG. 4 of this reference, as the composition of In increases, i.e., y approaches 0.5, the critical thickness drops dramatically. Turning now to FIG. 10 of this reference, it may be seen that Coleman has demonstrated that as the In concentration increases, the peak transition wavelength increases in a sub-linear fashion. As the In concentration approaches 0.5 the peak transition wavelength approaches about 1.20 xcexcm. If one was to extrapolate information from this graph for In concentrations greater than or equal to 0.5, one would come to the clear conclusion that a peak transition wavelength of 1.3 xcexcm is not obtainable while maintaining the InyGal-yAs layer within the critical thickness. Thus, while Coleman does provide a valuable teaching, he is unable to reach a 1.3 xcexcm peak transition wavelength.
The issue of strain compensation to increase the number of strained quantum wells which may be grown without misfit dislocations is frequently used in the art and is described by Zhang and Ovtchinnikov in an article entitled xe2x80x9cStrain-compensated InGaAs/GaAsP/GaInAsP/GaInP Quantum Well Lasers (xcexxcx9c0.98 xcexcm) Grown by Gas-Source Molecular Beam Epitaxy.xe2x80x9d Applied Physics Letters, vol. 62, pp. 1644-1646, 1993. The reader is also referred to U.S. Pat. No. 5,381,434 by Bhat and Zah.
The advantages of incorporating strain into the active region of a semiconductor laser were described by Yablonovitch in U.S. Pat. No. 4,804,639. Yablonovitch discloses active regions of InyGal-yAs grown on GaAs substrates, typically with yxcx9c0.5, and having a thickness preferably less than 100 xc3x85. He suggests the possibility of xe2x80x9cthe addition of counter-strain layers of GaP on either side of the active strained layer,xe2x80x9d but does not pursue this possibility. He goes on to perform numerical evaluations based on xe2x80x9can assumed set of numerical coefficients which are thought to be representative of a quaternary semiconductor with a band edge near the 1.5 xcexcm wavelength.xe2x80x9d The material is further assumed to have a strain of 3.7% and a thickness of 100 xc3x85 which was thought to be xe2x80x9cprobably the maximum permissible thickness for such a high strain.xe2x80x9d This strain for y=0.5 is calculated to be less than xcx9c40 xc3x85. Thus, although a  greater than 1.3 xcexcm emitting laser utilizing strained InGaAs on GaAs is indirectly suggested, no actual structure is specified and the parameters are not realistic.
In U.S. Pat. No. 5,060,030, Hoke describes improvements in electron mobility and electron saturation for use in high-electron-mobility transistors (HEMTs). He describes the use of strain compensation to increase the thickness or In concentration xe2x80x9cby approximately a factor of twoxe2x80x9d in a strained InGaAs layer grown on GaAs.
A strain-compensated heterostructure laser diode is described by Buchan et al. in U.S. Pat. No. 5,373,166. Buchan describes graded structures in the compressive and tensile strained quaternary layers with the strain magnitudes less than 1%. The thicknesses of the layers described are less than their conventional critical thicknesses.
Vawter et al., in an article entitled xe2x80x9cUseful Design Relationships for the Engineering of Thermodynamically Stable Strained-layer Structures,xe2x80x9d Journal of Applied Physics, vol. 65, pp. 4769-4773, 1989, describes approaches for engineering dislocation-free strained-layer structures. The article includes a methodology for calculating the xe2x80x9ccritical thicknessxe2x80x9d of structures comprising layers of differing lattice constants. The methodology is based upon a xe2x80x9creduced effective strainxe2x80x9d which is the sum of the strain-thickness products of all the layers divided by the total thickness of the layers. Based upon this xe2x80x9creduced effective strain,xe2x80x9d the xe2x80x9ccritical thicknessxe2x80x9d for the structures is then calculated from a critical thickness criterion, e.g., that introduced by Matthews and Blakeslee.
Asahi et al., in an article entitled xe2x80x9cNew III-V Compound Semiconductors TlInGaP . . . xe2x80x9d Japanese Journal of Applied Physics, describes the inclusion of the group-III element thallium (Tl) in III-V semiconductors for long-wavelength emission. Most of the discussion focuses on lasers emitting at wavelengths greater than 2 xcexcm on InP substrates, but it is stated that TlGaP lattice-matched to GaAs substrate has a bandgap emission of about 1.24 xcexcm. The extreme toxicity and hazardous nature of Tl, even after epitaxial growth is performed, makes it undesirable as a manufacturing material.
Very recently, it has been shown that adding nitrogen to InGaAs, actually decreases the peak transition energy and thereby increases the peak transition wavelength as described by Kondow et al., in an article entitled xe2x80x9cGaInNAs: A Novel Material for Long-Wavelength-Range Laser Diodes with Excellent High-Temperature Performance,xe2x80x9d Jpn. J. Appl. Phys., vol. 35, pp. 1273-1275, February 1996. The report suggests that it is difficult to grow high quality InGaAsN with very much N. A room temperature photo-luminescence spectrum of a 70 xc3x85 thick InyGal-yAsl-vNv/GaAs quantum well showed significant broadening even with only less than 1% N concentration for the group V semiconductor element. This corresponds to a value of v being less than 0.01. The peak transition wavelength of this semiconductor was 1.23 xcexcm. In the report, the authors state that their plan is to reach a 1.3 xcexcm device by increasing the N concentration to 1% while maintaining a 30% In concentration.
A hybrid approach to address the dual problem described earlier has been reported by Margalit et al., in an article entitled xe2x80x9cLaterally Oxidized Long Wavelength CW Vertical-Cavity Lasers,xe2x80x9d Applied Physics Letters, vol. 69, pp. 471-472, Jul. 22, 1996. In this work, two DBRs are grown on two separate GaAs substrates, while the active material is grown on a third substrate which comprises InP. The active material comprises seven compressively strained InGaAsP quantum wells clad by 300 nm of InP on each side. To assemble these materials, two processes are performed, each including the thermal fusion of two wafers and removal of one substrate. Then the resulting structure is processed by standard VCSEL processing methods.
Since VCSELs are presently the subject of intense research and development, a great deal of results and advancements are published periodically. The following is a list of documents which are relevant to the problem of extending emission wavelengths of semiconductor lasers or of producing 1.3 xcexcm through 1.55 xcexcm VCSELs.
Fisher et al., xe2x80x9cPulsed Electrical Operation of 1.5 xcexcm Vertical-Cavity Surface Emitting Lasers,xe2x80x9d IEEE Photonics Technology Letters, Vol. 7, No. 6, pp. 608-609, Jun. 6, 1995.
Uchiyama et al., xe2x80x9cLow Threshold Room Temperature Continuous Wave Operation of 1.3 xcexcm GaInAsP/InP Strained Layer Multiquantum Well Surface Emitting Laser,xe2x80x9d Electronics Letters, vol. 32, no. 11, pp. 1011-1013, May 23, 1996.
Hasenberg, xe2x80x9cLinear Optical Properties of Quantum Wells Composed of All-Binary InAs/GaAs Short-Period Strained-Layer Superlatives,xe2x80x9d Applied Physics Letters, vol. 58, no. 9, pp. 937-939, Mar. 4, 1991.
Fukunaga et al., xe2x80x9cReliable Operation of Strain-Compensated 1.06 xcexcm InGaAs/InGaAsP/GaAs Single Quantum Well Lasers,xe2x80x9d Applied Physics Letters., vol. 69, no. 2, pp. 248-250, Jul. 8, 1996
Kondow et al., xe2x80x9cGas-Source Molecular Beam Epitaxy of GaNx As 1-x Using a N Radical as the N Source,xe2x80x9d Jpn. J. Appl. Phys., vol. 33, pp. 1056-1058, Aug. 1 1994.
Shimomura et al., xe2x80x9cImproved Reflectivity of AlPSb/GaPSb Bragg Reflector for 1.55 xcexcm Wavelength,xe2x80x9d Electronics Letters, vol. 30, no. 25, pp. 2138-2139, Dec. 8, 1994.
Blum et al., xe2x80x9cWet Thermal Oxidation of AlAsSb Lattice Matched to InP for Optoelectronic Applications,xe2x80x9d Applied Physics Letters., vol. 68, no. 22, pp. 3129-3131, May 27, 1996.
Mirin, R. P., xe2x80x9c1.3 xcexcm Photoluminescence From InGaAs quantum dots on GaAs,xe2x80x9d Applied Physics Letters., vol. 67, no. 25, pp. 3795-3797, Dec. 18, 1995.
Thus, although the prior art therefore describes a variety of techniques useful in forming long-wavelength lasers on GaAs substrates, it fails to provide any specific example of a viable such structure, nor does it provide any range of parameters within which viable such structures may be fabricated, nor does it teach the construction of a viable such structure. Some references suggest the possibility of 1.3 xcexcm lasers on GaAs substrates, but provide unrealistic parameters and are several years old or more.
It is therefore an object of the present invention to provide an active region having a quantum well structure which may be utilized in lasers grown on GaAs substrates and which will provide an emission wavelength of at least 1.3 xcexcm. Extensive work on novel strained InGaAs/GaAs heterostructures have led to an unexpected conclusion which contradicts the conclusions reached in the prior art. It has been found that use of high-indium-content structures permits emission wavelengths of at least 1.3 xcexcm using active layers grown on GaAs substrates which do not exceed their critical thickness. Other techniques allow pseudomorphic growth of active layers above their nominal critical thickness. These techniques, carefully applied to newly-identified parameter spaces, allow the unexpected result of pseudomorphic structures grown on GaAs substrates which emit at 1.3 xcexcm and longer wavelengths. Parameter spaces are defined for viable structures grown on GaAs substrates and emitting at 1.3 xcexcm or longer. Specific examples of these viable structures are provided in the detailed description, below.
Several methods are used in novel ways with newly identified and viable parameters to decrease the peak transition energies of the pseudomorphic InGaAs/GaAs heterostructures. These techniques, taken separately or in combination, suffice to permit operation at wavelengths of 1.3 xcexcm or greater of light-emitting electro-optic devices. These methods or techniques, by example, include: (1) utilizing new superlattice structures having high In concentrations in the active region, (2) utilizing strain compensation to increase the usable layer thickness for quantum wells with appropriately high In concentrations, (3) utilizing appropriately small amounts of nitrogen (N) in the pseudomorphic InGaAsN/GaAs laser structure, and (4) use of nominal (111) oriented substrates to increase the usable layer thickness for quantum wells with appropriately high In concentrations. In all of the above techniques, gain offset may be utilized in VCSELs to detune the emission energy lower than the peak transition energy, by about 25 meV or even more, via appropriate DBR spacing. Gain offset may also be utilized in some forms of in-plane lasers. Increased temperature may also be used to decrease peak transition energy (and therefore the emission energy) by about 50 me V/100xc2x0 C. All these techniques are furthermore applicable to other material systems, for example, extending the emission wavelength for laser diodes grown on InP substrates.
It is a further object to provide a laser having a long life and being easily manufacturable.
It is yet another object to provide various techniques which may be utilized in combination with high In concentrations to reduce the peak transition energy of a device, having a GaAs substrate, to allow for an emission wavelength of 1.3 xcexcm or greater.
It is yet another object to provide a pseudomorphic superlattice structure on a GaAs substrate which reduces the peak transition energy sufficiently to allow for an emission wavelength of 1.3 xcexcm or greater.
According to one broad aspect of the present invention, there is provided a light emitting device having at least a substrate and an active region, the light emitting device comprising: the substrate having a substrate lattice constant between 5.63 xc3x85 and 5.67 xc3x85; the active region comprising at least one pseudomorphic light emitting active layer disposed above the substrate, the active layer comprising at least In, Ga, As and N, the active layer having a thickness equal to or less than a respective CT, where:
CT=(0.4374/f) [In(CT/4)+1], 
where f is an average lattice mismatch of the active layer normalized to a lattice constant of 5.65 xc3x85; the active layer having an average sum of In and Sb concentrations in the active layer at 16.5% or greater of a semiconductor material in the active layer and the nitrogen content less than 1% of a group V semiconductor material in the active region; and wherein the light emitting device has an emission wavelength of at least 1.3 xcexcm.
According to another broad aspect of the invention, there is provided a light emitting device having at least a substrate and an active region, the light emitting device comprising: the substrate comprising having a substrate lattice constant between 5.63 xc3x85 and 5.67 xc3x85; the active region comprising at least one pseudomorphic light emitting active layer disposed above the substrate, the active layer comprising at least In, Ga and As, the active layer having a thickness equal to or less than 1.25 times a respective CT, where:
CT=(0.4374/f) [In(CT/4)+1], 
where f is an average lattice mismatch of the active layer normalized to a lattice constant of 5.65 xc3x85; wherein the active layer comprises at least two strained layers, and a third layer disposed between the two strained layers, forming a superlattice where an average sum of In and Sb concentrations in the superlattice is 25% or greater of a semiconductor material in the active layer; and wherein the light emitting device has an emission wavelength of at least 1.3 xcexcm.
According to another broad aspect of the invention, there is provided a light emitting device having at least a substrate and an active region, the light emitting device comprising: the substrate comprising having a substrate lattice constant between 5.63 xc3x85 and 5.67 xc3x85; the active region comprising at least one pseudomorphic light emitting active layer disposed above the substrate, the active layer comprising at least In, Ga and As, the active layer having a thickness equal to or less than 1.25 times a respective CT, where:
CT=(0.4374/f) [In(CT/4)+1], 
where f is an average lattice mismatch of the active layer normalized to a lattice constant of 5.65 xc3x85; wherein the active layer comprises at least two strained layers, and a third layer disposed between the two strained layers, forming a superlattice where an average sum of In and Sb concentrations in the superlattice is greater than 25% of a semiconductor material in the active layer; a first conductive layer having a first conductivity type, the first conductive layer disposed in electrical communication with the active layer; a second conductive layer having a second conductivity type, the second conductive layer being disposed above the active layer and in electrical communication therewith; and electrical communication means for providing electrical current to the active layer; and wherein the light emitting device has an emission wavelength of at least 1.3 xcexcm.
According to another broad aspect of the invention, there is provided a light emitting device having at least a substrate and an active region, the light emitting device comprising: the substrate having a substrate lattice constant between 5.63 xc3x85 and 5.67 xc3x85; a first strained layer having a lattice constant smaller than the substrate lattice constant and being disposed between the substrate and the active region; the active region comprising at least one pseudomorphic light emitting active layer disposed above the substrate, the active layer comprising at least In, Ga and As, the active layer comprises at least two strained layers, and a third layer disposed between the two strained layers, the active layer having a thickness equal to or less than 80 xc3x85; and wherein the light emitting device has an emission wavelength of at least 1.3 xcexcm.
According to another broad aspect of the invention, there is provided a light emitting device having at least a substrate and an active region, the light emitting device comprising: the substrate having a substrate lattice constant between 5.63 xc3x85 and 5.67 xc3x85; a first strained layer having a lattice constant smaller than a substrate lattice constant and being disposed between the substrate and the active region; the active region comprising at least one pseudomorphic light emitting active layer disposed above the substrate, the active layer comprising at least In, Ga and As; the active layer having a concentration of In and Sb of 25% or greater of a semiconductor material in the active layer, the active layer having a thickness greater than CT and less than 2.5 times CT for a given material, where:
CT=(0.4374/f) [In(CT/4)+1], 
where f is an average lattice mismatch of the active layer normalized to a lattice constant of 5.65 xc3x85; wherein the light emitting device has an emission wavelength of at least 1.3 xcexcm.
According to another broad aspect of the invention, there is provided a light emitting device having at least a substrate and an active region, the light emitting device comprising: the substrate comprising having a substrate lattice constant between 5.63 xc3x85 and 5.67 xc3x85; the active region comprising at least one pseudomorphic light emitting active layer disposed above the substrate, the active layer comprising at least In, Ga, As and N, the active layer having a thickness equal to or less than a respective CT, where:
CT=(0.4374/f) [In(CT/4)+1], 
where f is an average lattice mismatch of the active layer normalized to a lattice constant of 5.65 xc3x85; wherein the active layer comprises at least two strained layers, and a third layer disposed between the two strained layers, forming a superlattice having a nitrogen content of at least 0.01% of a group V semiconductor material in the active region; and wherein the light emitting device has an emission wavelength of at least 1.3 xcexcm.
According to another broad aspect of the invention, there is provided a light emitting device having at least a substrate and an active region, the light emitting device comprising: the substrate comprising having a substrate lattice constant between 5.63 xc3x85 and 5.67 xc3x85; a first strained layer disposed between the substrate and the active region, the first strained layer having a first accumulated strain and a first critical accumulated strain associated therewith, the first accumulated strain being less than the first critical accumulated strain; the active region comprising at least one pseudomorphic light emitting active layer disposed above the substrate, the active layer comprising at least In, Ga, As and N, the active layer comprising at least two strained layers, and a third layer disposed between the two strained layers, forming a superlattice having a nitrogen content of at least 0.01% of a group V semiconductor material in the active region, the active layer having a second accumulated strain and a second critical accumulated strain associated therewith, the algebraic sum of the first and second accumulated strains being less than the second critical accumulated strain; wherein the first and second critical accumulated strain for a given material equal a strain of the material multiplied by CT for a given material, where:
CT=(0.4374/f) [In(CT/4)+1], 
where f is an average lattice mismatch of the material normalized to a lattice constant of 5.65 xc3x85; and wherein the light emitting device has an emission wavelength of at least 1.3 xcexcm.
According to another broad aspect of the invention, there is provided a light emitting device having at least a substrate and an active region, the light emitting device comprising: the substrate comprising having a substrate lattice constant between 5.63 xc3x85 and 5.67 xc3x85; a first strained layer having a lattice constant smaller than the substrate lattice constant and being disposed between the substrate and the active region; the active region comprising at least one pseudomorphic light emitting active layer disposed above the substrate, the active layer comprising at least In, Ga, As and N, the active layer comprises at least two strained layers, and a third layer disposed between the two strained layers, the active layer having a nitrogen concentration of at least 0.01% of a group V semiconductor material in the active layer, the active layer having a thickness equal to or less than 175 xc3x85; and wherein the light emitting device has an emission wavelength of at least 1.3 xcexcm.
According to another broad aspect of the invention, there is provided a light emitting device having at least a substrate and an active region, the light emitting device comprising: the substrate comprising having a substrate lattice constant between 5.63 xc3x85 and 5.67 xc3x85; the substrate comprising having a substrate lattice constant between 5.63 xc3x85 and 5.67 xc3x85; a first strained layer having a lattice constant smaller than the substrate lattice constant and being disposed between the substrate and the active region; the active region comprising at least one pseudomorphic light emitting active layer disposed above the substrate, the active layer comprising at least In, Ga, As and N, the active layer comprises at least two strained layers, and a third layer disposed between the two strained layers, the active layer having a nitrogen concentration of at least 0.01% of a group V semiconductor material in the active layer, the active layer having a thickness equal to or greater than CT for a given material, where:
CT=(0.4374/f) [In(CT/4)+1], 
where f is an average lattice mismatch of the active layer normalized to a lattice constant of 5.65 xc3x85; and wherein the light emitting device has an emission wavelength of at least 1.3 xcexcm.
According to another broad aspect of the invention, there is provided a light emitting device having at least a substrate and an active region, the light emitting device comprising: the substrate comprising having a substrate lattice constant between 5.63 xc3x85 and 5.67 xc3x85; a first strained layer disposed between the substrate and the active region, the first strained layer having a first accumulated strain and a first critical accumulated strain associated therewith, the first accumulated strain being less than the first critical accumulated strain; the active region comprising at least one pseudomorphic light emitting active layer disposed above the substrate, the active layer comprising at least In, Ga, As and N, the active layer comprises at least two strained layers, and a third layer disposed between the two strained layers, the active layer having an average sum of In and Sb concentrations in the superlattice at 33% or greater and the nitrogen content of at least 0.01% of a group V semiconductor material in the active region, the active layer having a second accumulated strain and a second critical accumulated strain associated therewith, the algebraic sum of the first and second accumulated strain being less than the second critical accumulated strain; wherein the first and second critical accumulated strain for a given material equals a strain of the material multiplied by CT for a given material, where:
CT=(0.4374/f) [In(CT/4)+1], 
where f is an average lattice mismatch of the active layer normalized to a lattice constant of 5.65 xc3x85; and wherein the light emitting device has an emission wavelength of at least 1.3 xcexcm.
According to another broad aspect of the invention, there is provided a light emitting device having at least a substrate and an active region, the light emitting device comprising: the substrate comprising having a substrate lattice constant between 5.63 xc3x85 and 5.67 xc3x85; a first strained layer having a lattice constant smaller than the substrate lattice constant and being disposed between the substrate and the active region; the active region comprising at least one pseudomorphic light emitting active layer disposed above the substrate, the active layer comprising at least In, and Ga, the active layer comprising at least two strained layers, and a third layer disposed between the two strained layers, the active layer having a second accumulated strain and a second critical accumulated strain associated therewith, the algebraic sum of the first and second accumulated strains being less than the second critical accumulated strain; and wherein the light emitting device has an emission wavelength of at least 1.3 xcexcm.
According to another broad aspect of the invention, there is provided a light emitting device having at least a substrate and an active region, the light emitting device comprising: the substrate comprising having a substrate lattice constant between 5.63 xc3x85 and 5.67 xc3x85; a first strained layer disposed between the substrate and the active region, the first strained layer having a first accumulated strain and a first critical accumulated strain associated therewith, the first accumulated strain being less than the first critical accumulated strain; the active region comprising at least one pseudomorphic light emitting active layer disposed above the substrate, the active layer comprising at least In, Ga and As, the active layer comprises at least two strained layers, and a third layer disposed between the two strained layers, forming a superlattice having an average sum of In and Sb concentrations in the superlattice at 25% or greater of a semiconductor material in the active layer, the active layer having a second accumulated strain and a second critical accumulated strain associated therewith, the second accumulated strain being less than the second critical accumulated strain; wherein the first and second critical accumulated strain for a given material equals a strain of the material multiplied by CT for a given material, where:
CT=(0.4374/f) [In(CT/4)+1], 
where f is an average lattice mismatch of the active layer normalized to a lattice constant of 5.65 xc3x85; and wherein the light emitting device has an emission wavelength of at least 1.3 xcexcm.
According to another broad aspect of the invention, there is provided a light emitting device having at least a substrate and an active region, the light emitting device comprising: the substrate comprising having a substrate lattice constant between 5.63 xc3x85 and 5.67 xc3x85 and having a growth plane which has an orientation within 15xc2x0 of (111); the active region comprising at least one pseudomorphic light emitting active layer disposed above the substrate, the active layer comprising at least In, Ga and As, the active layer having a thickness equal to or less than twice a respective CT, where:
CT=(0.4374/f) [In(CT/4)+1], 
where f is an average lattice mismatch of the active layer normalized to a lattice constant of 5.65 xc3x85; wherein the active layer has an average sum of In and Sb concentrations of equal to or greater than 25% or greater of a semiconductor material in the active layer; and wherein the light emitting device has an emission wavelength of at least 1.3 xcexcm.
Other objects and features of the present invention will be apparent from the following detailed description of the preferred embodiment.